40 BOTANY 
land has been dug or ploughed in late autumn, weed 
seedlings do not appear in any quantity till the winter 
frosts are over. No farmer thinks of sowing turnips at 
midwinter, while the tender French bean should not 
be put into the soil till all risk of frost is over. There 
+s a maximum temperature above which, and a 
minimum below which germination will not take place. 
There is, moreover, an optimum temperature at which 
it proceeds most vigorously. These temperatures vary 
for different plants. In most plants cultivated in 
temperate regions vital activity is suspended below 
40° F., while the maximum temperature is, In most 
cases, about 115° F. The optimum is between 80° and 
95° according to the plant, wheat being 84° and 
maize 93°. 
Experiment 2.—This experiment, which should be 
made in the summer time, can be carried out only 
where a refrigerator is working in the neighbourhood, 
and i¢e can, therefore, without difficulty, be obtained 
at a low cost. Place a block of ice, (about six-penny: 
worth will be enough) in a fair-sized box, pack it below 
and at the sides with moist sawdust, and lay upon it 
various kinds of seeds wrapped in a piece of damp 
cloth. Now cover the whole to the depth of several 
inches with more of the damp sawdust. The seeds are 
wrapped in cloth to prevent their being seattered as the 
ice melts. As required, renew the ice every few days for 
a fortnight. There will be no germination. This shows 
that seeds will not germinate at a temperature below 
freezing point. That the seeds are not killed is shown 
by the fact that they germinate when ice is no longer 
supplied. Experiments to discover the maximum, 
minimum, and optimum temperatures, by means of 
germination experiments carried out in chambers 
constantly kept at a certain temperature, would be far 
more instructive than the foregoing, but are obviously 
beyond the scope of an elementary course. 
